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Oceanic movements, site fidelity and deep diving in harbour porpoises from Greenland show limited behavioural similarities to North Sea harbour porpoise population

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Harbour porpoises Phocoena phocoena are common in continental shelf areas of the North Atlantic, but little information is available on their occurrence outside coastal areas. In this study, 30 harbour porpoises were actively caught in West Greenland and instrumented with satellite transmitters to document their seasonal movements and diving behaviour. The porpoises displayed long-range oceanic movements within the North Atlantic, especially during winter/spring where they moved over areas with water depths >2500 m. While offshore, 2 females demonstrated an average maximum dive depth of 248 m, with the deepest dive reaching 410 m. This behaviour is in contrast to 71 porpoises tagged in Danish waters of the North Sea which did not leave the continental shelf but showed a preference for areas with shallow waters year round, even when at the edge of the continental shelf where greater depths were available. Six tags from Greenland transmitted long enough (up to 3 yr) to demonstrate extensive movements and strong site fidelity to the tagging site in West Greenland the following summer. This study documents that harbour porpoises use oceanic habitats and can dive to depths that enable mesopelagic foraging, while repeatedly demonstrating summer site fidelity to coastal areas.
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MARINE ECOLOGY PROGRESS SERIES
Mar Ecol Prog Ser
Vol. 597: 259–272, 2018
https://doi.org/10.3354/meps12588 Published June 11
INTRODUCTION
Harbour porpoises Phocoena phocoena are
thought to be closely associated with continental
shelf areas (depth <200 m) (Read & Westgate 1997),
where most surveys of harbour porpoise abundance
have been conducted (Forney & Barlow 1998, Ham-
mond et al. 2013, Hansen & Heide-Jørgensen 2013).
Little in formation is available on their distribution
outside coastal areas despite their potential for mov-
ing over large areas (Johnston et al. 2005). Com-
pared to other toothed whales, harbour porpoises
are referred to as living ‘life in the fast lane’ due to
their early maturity, high pregnancy rate and short
lifespan (Read & Hohn 1995), and moreover, harbour
porpoises have a high energy demand. Their small
size gives them a large surface to body volume ratio
(see Kleiber 1947), which induces higher energy
loss through radiation and thermal conduction com-
pared to larger ceta ceans. Despite large seasonal
variations in the blubber layer that serves as an
adaptive mechanism to cope with water tempera-
tures from subarctic to temperate habitats (Lockyer
et al. 2003a), living in these habitats requires por-
poises to locate high densities and predictability of
prey to maintain basal metabolism and blubber
insulation (Koopman 1998, Wisniewska et al. 2016).
To keep up with the energy demand, they exploit a
wide range of prey species, mainly fish, but also
cephalopods and copepods, and thus porpoises are
© Inter-Research 2018 · www.int-res.com*Corresponding author: nel@ghsdk.dk
Oceanic movements, site fidelity and deep diving in
harbour porpoises from Greenland show limited
similarities to animals from the North Sea
Nynne H. Nielsen1,2,*, Jonas Teilmann2, Signe Sveegaard2, Rikke G. Hansen1,
Mikkel-Holger S. Sinding1, Rune Dietz2, Mads Peter Heide-Jørgensen1
1Greenland Institute of Natural Resources, 3900 Nuuk, Greenland
2Aarhus University, Department of Bioscience, 4000 Roskilde, Denmark
ABSTRACT: Harbour porpoises Phocoena phocoena are common in continental shelf areas of the
North Atlantic, but little information is available on their occurrence outside coastal areas. In this
study, 30 harbour porpoises were actively caught in West Greenland and instrumented with satel-
lite transmitters to document their seasonal movements and diving behaviour. The porpoises dis-
played long-range oceanic movements within the North Atlantic, especially during winter/spring
where they moved over areas with water depths > 2500 m. While offshore, 2 females demonstrated
an average maximum dive depth of 248 m, with the deepest dive reaching 410 m. This behaviour
is in contrast to 71 porpoises tagged in Danish waters of the North Sea which did not leave the con-
tinental shelf but showed a preference for areas with shallow waters year round, even when at the
edge of the continental shelf where greater depths were available. Six tags from Greenland trans-
mitted long enough (up to 3 yr) to demonstrate extensive movements and strong site fidelity to the
tagging site in West Greenland the following summer. This study documents that harbour por-
poises use oceanic habitats and can dive to depths that enable mesopelagic foraging, while
repeatedly demonstrating summer site fidelity to coastal areas.
KEY WORDS: Argos satellite tracking · Oceanic movements · North Atlantic · Danish waters ·
Diving behaviour · Mesopelagic prey · Habitat selection
Resale or republication not permitted without written consent of the publisher
Mar Ecol Prog Ser 597: 259–272, 2018
260
often referred to as opportunistic feeders. However,
in some cases their choice of prey may be depend-
ent on its caloric content (Fontaine et al. 1994, Lock-
yer et al. 2003b, Heide-Jørgensen et al. 2011, Spitz
et al. 2012, Leo pold et al. 2015, Andreasen et al.
2017; for a review, see Santos & Pierce 2003).
Harbour porpoises are believed to avoid sea ice
and sub-zero temperatures, and they seem to prefer
certain sea surface temperatures (SSTs) (Wingfield et
al. 2017). Other environmental variables such as dis-
tance to coast, bottom salinity and bathymetry also
affect the distribution of harbour porpoises (Edrén et
al. 2010, Gilles et al. 2011).
Mitochondrial DNA studies have suggested that
the Northwest and Northeast Atlantic populations
are somewhat discrete, and that exchange of genes
across the Atlantic rarely occurs (Rosel et al. 1999;
see Andersen 2003 for a review). Porpoises from
West Greenland are believed to be genetically sepa-
rate from other populations in both the Northeast
(Norway, North Sea and Ireland; Andersen et al.
2001) and the Northwest and Central Atlantic (Gulf
of Maine, Gulf of St. Lawrence, Newfoundland and
Iceland, Tolley et al. 2001). However, porpoises from
West Greenland are thought to be most closely re -
lated to porpoises from the Gulf of St. Lawrence
(Rosel et al. 1999).
Information on the ecology of West Greenland har-
bour porpoises includes studies on the abundance,
distribution and life history of animals on the conti-
nental shelves (Teilmann & Dietz 1998, Lockyer et al.
2001, 2003b, Heide-Jørgensen et al. 2011, Hansen &
Heide-Jørgensen 2013). Porpoises from West Green-
land use the productive waters of the West Green-
land shelf to prey on a variety of species, with capelin
Mallotus villosus, Arctic cod Boreogadus saida and
cephalopods being the main prey (Heide-Jørgensen
et al. 2011). Harbour porpoises are subject to hunting
by humans year round in Greenland, although most
of the hunt takes place in August−October, suggest-
ing that these months also reflect the main occur-
rence of harbour porpoises on the coastal shelves
(NAMMCO 2013). The highest densities of porpoises
in Greenland are located on the southwest coast
(Teilmann & Dietz 1998), and this is also where most
of the hunting of porpoises takes place.
To gain insight into the ecology and population dis-
creteness of harbour porpoises in West Greenland,
30 harbour porpoises were instrumented with Argos
satellite transmitters between 2012 and 2014. The
transmitters provided information on the porpoise
movements and habitat preferences; these data were
compared to those of 71 harbour porpoises from
the North Sea population tagged in Danish waters
between 1997 and 2015.
MATERIALS AND METHODS
Capture and handling of harbour porpoises
Thirty harbour porpoises were actively live-cap-
tured on the continental shelf ca. 50 km south-west of
Maniitsoq, West Greenland, in July 2012 (n = 2); July,
September and October 2013 (n = 6, 3 and 5, respec-
tively); and July 2014 (n = 14). Two 6.30 m fibreglass
dinghies with 150 hp outboard engines were used to
spot and catch the porpoises. The boats were oper-
ated by 2 experienced local hunters. One boat (net
launch boat, with one person) carried a monofila-
ment surface gill net, 5 m deep and 50 m long with
stretched mesh size of 20 cm. The nets were fitted
with an upper float line and a bottom lead line to
keep the nets vertically oriented at the surface of the
water. When a porpoise was spotted, the gill net was
quickly launched and the porpoise was herded by
the second boat (herd boat, with 3 persons). The
boats kept a close lookout for the porpoise while the
herd boat followed it and kept it on the starboard side
until herded into the net. Porpoises avoided the boats
and thus reacted to being herded in a specific direc-
tion. As soon as entanglement of the porpoise was
observed by movements at the float line or the ani-
mal being visible at the surface, both boats quickly
went to the net, where the engines were stopped to
prevent further stress of the animal. The animal was
then removed from the net and lifted into the boat on
a foam pad where it was dosed regularly with sea
water. This was done to prevent the skin from drying
and to keep the animal breathing regularly. The
duration of the herding was on average 15−20 min.
On a few occasions, 2 porpoises were caught at the
same time and one boat kept the second porpoise at
the surface in the net until the other boat had fin-
ished instrumenting the first animal. Information on
sex, mass and length was collected before release
(Table 1). The handling time was on average 5 min,
and all porpoises quickly swam away with regular
surfacings upon release.
The 71 porpoises tagged in Danish waters were
caught incidentally in pound nets. The fisherman
would then contact the research team that reached
the net within 24 h. Information on the 71 harbour
porpoises caught in Danish waters is provided in
Table S1 in the Supplement at www.int-res.com/
articles/ suppl/ m597 p259 _ supp. pdf.
Nielsen et al.: Behavioural differences between porpoise populations 261
PTT ID Type Deployment Tag Sex Length Body Initial shelf duration Maximum dive Median maximum Days
longevity (cm) mass (d) prior to depth (m) dive depth (m) with dive
(d) (kg) offshore movement Shelf Offshore Shelf Offshore data
7617a,b SPLASH 25 Jul 121476 F 128 NA 2 168 390 168 240 104
7618a,b SPLASH 25 Jul 121422 F 156 NA 7 410 410 265 261 150
20160aSPOT 5 7 Jul 13 243 M 140 43 170
20164a,b SPOT 5 7 Jul 13 376dF 128 41 163
20165aSPOT 5 10 Jul 13 393 F 115 28 204
20166aSPOT 5 7 Jul 13 227 M 136 39 139
20167aSPOT 5 10 Jul 13 146 F 124 31 2
20169aSPOT 5 6 Jul 13 268 M 120 31 218
21791a,b SPOT 5 16 Jul 14 649 F 111 NA 179
21792a,b SPOT 5 30 Jul 14 1047 M 130 NA 71
21793 SPOT 5 17 Jul 14 95 F 123 NA
22849 MK10c25 Sep 13219 F NA 30 150 100 5
22850 MK10 3 Oct 13 71 F 140 43 15 250 250 200 150 10
22853aMK10c25 Sep 13229 F 166 NA 7 300 250 200 150 3
22854 MK10 22 Jul 14 138 M 130 NA 350 200 111
27261 MK10c2 Oct 13364 M 125 33 250 150 16
27262 MK10c2 Oct 13316d,e F 150 NA 300 275 4
37227aMK10c2 Oct 13446 F 129 47 4 100 150 100 75 3
37228 MK10c2 Oct 13466 F 150 53 200 150 15
37235 MK10c27 Sep 13 67 F 126 35 250 150 8
93096 MK10 16 Jul 14 130 M 120 NA 400 200 109
93100 MK10 22 Jul 14 170dM 145 NA 400 250 20
93102 MK10 16 Jul 14 148 M 149 NA 400 200 121
228491 MK10 22 Jul 14 146 F 123 NA 350 200 99
228501 MK10 17 Jul 14 137 F 125 NA 350 200 119
272611 MK10 16 Jul 14 130 M 152 NA 350 200 118
272621 MK10 22 Jul 14 134dM 140 NA 400 200 104
372271aSPOT 5 11 Jul 14 478 M 127 39 193
372281a,b SPOT 5 18 Jul 14 1043 M 120 NA 216
372351 MK10 16 Jul 14 136 F 135 NA 350 200 118
aAnimals that moved offshore (< 200 m for 20 out of 30 d)
bAnimals that returned to the tagging area
cTransmitters with a battery defect
dPorpoises shot by a hunter
eTags that ended their transmission due to the porpoise being shot
Table 1. Basic information on 30 harbour porpoises instrumented with satellite transmitters in West Greenland 2012−2014. Superscript numbers indicate porpoises
caught together (presumed mother/calf pairs). The exact daily maximum dive depth was recorded for PTT ID 7617 and 7618. The remaining dive depths were calcu-
lated from the binned dive depths of the Mk10 tags. PTT ID: platform transmitter terminal identification; NA: data not available
Mar Ecol Prog Ser 597: 259–272, 2018
Instrumentation with satellite transmitters
The animals in West Greenland were instrumented
with 3 types of Argos satellite-linked transmitters
(2 SPLASH: 80L × 19W × 49H mm, 76 g; 18 Mk10:
108 × 41 × 21 mm; 75 g; and 10 SPOT5 tags: 81 × 19 ×
51 mm; 49 g), from Wildlife Computers, modified for
fin-mounted use on harbour porpoises. The tags
were attached to the dorsal fin using 3 delrin pins
(5 mm diameter) covered by silicone tubing, increas-
ing the total diameter of the pins to 6 mm. These pins
were attached to the tag by stainless steel nuts and
pushed through holes drilled in the fin with a steril-
ized 7 mm cork borer mounted on a battery drill. The
extra milli metre facilitated room for tissue swelling
and re duced the risk of pressure necrosis around the
pins. The pins were secured on the opposite side of
the fin to a backing plate (2 mm rubber conveyer belt
) with stainless steel nuts, and the distal part of the
pins was flattened to secure the nuts. In order to col-
lect data for as long as possible, the tags were not
designed to release from the animal, as used for other
porpoises instrumented with fin-mounted satellite
transmitters (Teilmann et al. 2007). Both the tag and
backing plate were lined with closed-cell neoprene
to prevent abrasion. For details on the tagging proce-
dure of the Danish porpoises, see Teilmann et al.
(2007) and Sveegaard et al. (2011).
Data analysis
The instrumented SPOT5 tags were designed to
provide only the location of the porpoise, while the
SPLASH and Mk10 tags provided information on
location, time and pressure (depth). These were sam-
pled at a default rate of every second, and stored in
6 h summary histograms that were relayed to the
satellite during the following 24 h. The histograms
were sampled in four 6 h bins: 01:0007:00, 07:00
13:00, 13:00–19:00 and 19:0001:00 h. For SPLASH
tags, the user-defined intervals were 0 m, then 5 m
bins up to 30 m, then 10 m bins up to 90 m and then
>90 m. For Mk10 tags deployed in 2013, the intervals
were 5, 10, 25 and 50 m, then 50 m bins up to 500 m
and then >500 m. For Mk10 tags deployed in 2014,
the intervals were 1, 2, 10, 25 and 50 m, then 50 m
bins up to 450 m and then >450 m. In addition, the
SPLASH tags also provided information on the maxi-
mum dive depth (m) during the past 24 h. All tags
sampled and transmitted histograms of maximum
depth for each dive in 14 user-defined intervals. In
order for the battery to hold the charge as long as
possible, the tags were programmed to wake up and
record data every fourth day (SPOT5 PTT IDs: 21791,
21792, 21793, 372271, 372281) and every second day
for all remaining tags.
The tags were limited with regard to data collec-
tion, and the 14 pre-defined bin intervals were based
on the assumed dive range of the porpoises. As no
one had tagged harbour porpoises in West Green-
land prior to this study, the first tags (2012) were pro-
grammed following previous tag deployments on
porpoises in Danish waters (Teilmann et al. 2007).
For the second deployment (2013) it was clear that
porpoises in West Greenland were diving to greater
depths and thus the bin intervals were changed to
cover their diving range. For the last deployment
(2014) the bin intervals were altered to collect data
on porpoise surface time for abundance estimations
in West Greenland (Hansen et al. in press). In order
to register the capacity for deep dives, the last bin
intervals (>90, > 500 and > 450 m, respectively) regis-
tered all potential dives exceeding this depth.
The first Mk10 tags deployed in 2013 (n = 8, Table 1)
were acknowledged by the manufacturers to be
subjected to a defect, which resulted in sudden dis-
charging of the batteries leading to low tag lifespan.
The mean lifespan of these 8 tags was 47 d (range:
16−71 d, SD = 23) and significantly shorter than the
other Mk10 tags (n = 10) that transmitted for a mean
of 141 d (range: 130−170 d, SD = 12, 2-sample t-test,
t16, p < 0.0001).
The Argos positions of the porpoises from West
Greenland were obtained from the Argos satellite
data processing system, filtered using the R (version
3.3.3, R Development Core Team 2008) package
‘argosfilter’ (Freitas et al. 2008), while the compara-
ble SAS-routine (SAS 9.3), Argos-Filter v7.03 (Dou-
glas 2006) was used for porpoises tagged in Danish
waters. The accuracy of locations was assessed by
Argos location codes (LC) B, A and 03, and an aver-
age position was calculated for all good-quality posi-
tions (LC = 1–3) for each transmission day. If no good-
quality positions were available, low-quality posi-
tions (LC = 0, A and B) were used for the subsequent
analysis. The ranging areas for the 2 porpoise popu-
lations were estimated by creating minimum convex
polygons (MCPs) using the freeware QGIS (version
2.18, QGIS Development Team 2017). To visually dis-
play the density of harbour porpoises in West Green-
land and the North Sea, the ‘Heatmap’ plugin in
QGIS 2.18 was used and 50% and 95% kernel densi-
ties were added. ‘Heat map’ uses kernel density esti-
mation to create a density raster from the porpoise
positions, calculated based on the number of posi-
262
Nielsen et al.: Behavioural differences between porpoise populations 263
tions within a grid cell. The density was weighted by
individual PTT ID. The radius (band-width) was
based on inspection of kernel contours during tests of
alternate band-width as recommended by Kie (2013).
Upon testing, the radius were set to 90 000 for por-
poises tagged in West Greenland and 50 000 for por-
poises tagged in Denmark, and the cell size was set
to 50 km2and 4 km2, respectively.
The bathymetry (Jakobsson et al. 2012) at each
porpoise position was calculated in ArcGIS. For the
porpoises tagged in Greenland, we used map projec-
tion UTM Zone 25N, WGS84, and for the porpoises
tagged in Denmark, we used map projection UTM
Zone 32N, WGS84. XLSTAT version 2014.6.01 was
used for statistical analysis. To analyse porpoise
movement in relation to sea ice, the monthly sea
ice distribution in 2013, 2014, 2015 and 2016 was ob-
tained from sea ice charts of the Labrador Sea and off
the coast of Newfoundland (Danish Meteorological
Institute 2017) and plotted in QGIS 2.18. The charts
measured the sea ice concentrations in tenths in the
following categories; <1/10 = open water, 1/10 3/10
= very open drift ice, 4/10 6/10 = open drift ice,
7/108/10 = close drift ice, 9/10 = very close drift ice,
10/10 = fast ice. Only sea ice concentrations >1/10
were included in the analysis. Sea surface tempera-
tures (SST) were downloaded from Copernicus ((http://
marine.copernicus.eu/services-portfolio/ access-to-
products/) and analysed in QGIS 2.18.
The monthly maximum sea ice distribution for
West Greenland was visually inspected for the
years 2013−2016, to cover the movement of the
instrumented porpoises, and the maximum ice dis-
tribution was located and compared with the distri-
bution of the instrumented harbour porpoises in the
same month. Similarly, North Atlantic SSTs were
obtained and added to the maps for inter-annual
comparison.
Fig. 1. Minimum convex polygons (MCP) of the total area covered by 30 harbour porpoises tagged in West Greenland (Gl, green
line), and 71 porpoises tagged in Denmark (Dk, blue line). Small dots represent all recorded satellite positions of porpoises tagged
in West Greenland (green) and Denmark (blue) for the duration of the tags. Larger dots highlight the positions of porpoises tagged
in West Greenland in May (green), June (yellow), July (pink) and August (red), thus illustrating the return of porpoises to the East
or West Greenland shelf over this period. The dashed line indicates the limit of the North Atlantic Ocean defined in this study
Mar Ecol Prog Ser 597: 259–272, 2018
264
Of the 71 porpoises tagged in Denmark, 30 were
tagged off Skagen in the Skagerrak (Fig. 1, Table S1).
These individuals are believed to belong to a large
population residing in the North Sea and adjacent
waters. The remaining 41 porpoises were tagged in
the Kattegat or the Danish Belt Seas; however, only
locations north of the Kattegat were included in the
data comparison (Sveegaard et al. 2015). The depths
in the Kattegat and the Belt Seas are generally <50 m
and were considered less comparable to the depths
in West Greenland.
All data used in analyses were tested for normality
using a Shapiro-Wilk test and homogeneity of vari-
ance using Levene’s test. Subsequently, bathymetry
at the position of all 30 tagged harbour porpoises
from West Greenland was analysed using a Kruskal-
Wallis test followed by a post-hoc Dunn’s comparison
test, and the dive depth of 2 female harbour por-
poises was analysed using a 1-way ANOVA followed
by a post hoc Tukey comparison test. Dive depth
based on the median was analysed using a Mann-
Whitney U-test. Student’s 2-sample t-test was used
for comparing travel rate (km d−1) and maximum dive
depth of 2 female harbour porpoises when moving
inshore and offshore. p-values less than 0.05 are con-
sidered significant.
RESULTS
Movement, bathymetry and dive depth
West Greenland harbour porpoises
Thirty satellite tags deployed on harbour porpoises
provided information for an average of 250 d (Table 1,
range: 16−1047 d, SD = 267) covering a total com -
bined MCP area of 4 144 749 km2(Fig. 1). The kernel
home range (50%) was concentrated around the
continental shelf off West Greenland (Fig. 2). The ker-
nel utilisation range (95%) covered a much larger
area including a large part of the North Atlantic. The
porpoises tagged in West Greenland were categorised
Fig. 2. Heatmaps (QGIS 2.18) for harbour porpoises tagged in Denmark and West Greenland, with kernel home ranges
(50%, Gl: dark green, Dk: dark blue) and kernel utilisation range (95%, Gl: green, Dk: blue) and minimum convex polygons
(Gl: green, Dk: blue). KDE: kernel density estimate
Nielsen et al.: Behavioural differences between porpoise populations
as being offshore when they had left the continental
shelf areas (water depths of <200 m) for 20 out of 30 d.
This criterion was developed in order to allow for
some temporary crossing of the continental shelf area.
Using these criteria, 15 of the porpoises left the West
Greenland shelf areas on average 106 d (range: 2−
218 d, SD = 91) after deployment: 12 left between Oc-
tober and February, 2 left in July, and 1 left in August.
These 15 porpoises (60% females: 1 calf, 5 subadults,
and 3 adults, and 40% males: 3 subadults and 3
adults, see Lockyer et al. 2003b for age classification)
transmitted for an average of 394 d (range: 29−1047 d,
SD = 317) while they moved south or west into deeper
waters of Davis Strait, the Labrador Sea and the North
Atlantic (Fig. 1). However, none of the porpoises en-
tered the shelf areas of eastern Canada or Iceland. Of
the 15 animals that went offshore, 4 ended their trans-
missions before their winter/spring (January−June)
movement could be identified. Of the remaining 11
porpoises, 10 moved to offshore areas in the North
Atlantic (defined as being south of 60° N, Fig. 1)
throughout winter/spring and went as far south as
45° N and as far west as 18° W, and ended their trans-
missions on average after 515 d (range: 227− 1047 d,
SD = 305). The 15 harbour porpoises that remained on
the West Greenland shelf areas ended their transmis-
sions in waters off Southwest Greenland or the north-
eastern part of the Labrador Sea on average after
106 d (range: 16−170 d, SD = 48).
The average water depth within the total range of
all porpoises tagged in West Greenland was 2936 m
(range: 8−4654 m, SD = 1007); however, the water
depth of the areas that the porpoises preferred varied
significantly between months (H11 = 355.04, p <
0.0001, Fig. 3A). Porpoises moved into areas with
deeper water depths in autumn/early winter (aver-
age of 2486 m in August−December, SD = 420)
compared to winter/early spring (average of 599 m in
January−June, SD = 277, p < 0.0001).
Two female porpoises (instrumented with SPLASH
tags, Table 1) that moved offshore provided a daily
maximum dive depth for 104 and 150 d, respectively.
265
Fig. 3. Preferred monthly average bathymetry for (A) 30 harbour porpoises from West Greenland and (B) for 71 porpoises from
the North Sea/Skagerrak. The central horizontal line in the boxes is the median and the lower and upper limits of the box are
the first and third quartiles, respectively. The upper and lower bounds of whiskers represent the 95 % confidence interval, and
blue dots indicate the minimum and maximum bathymetry range for each month. Number of animals (n) and median water
depth (mdn) is given for each month. In (B), 2 outliers in January (841 m) and December (658 m) are indicated by asterisks
Mar Ecol Prog Ser 597: 259–272, 2018
They dived to a mean daily maximum depth of 252 m
(range: 114−410 m, SD = 66, Table 1) with a record deep
dive of 410 m. They dived significantly deeper (F11,253 =
12.191, p < 0.0001) in November, December, February
and March (p < 0.0001) and significantly deeper when
moving into offshore waters (p < 0.05).
Maximum dive depth from the Mk10 tags (n =
17) was obtained using the daily maximum depth
bin visited by the porpoise (Table 1). The median
dive depth of all 17 porpoises was 200 m (range:
50− 400 m). Three of the 17 porpoises moved off-
shore, and their maximum dive depth was signifi-
cantly shallower when they were offshore than
when they were at the shelf areas (150 and 200 m,
respectively, Mann-Whitney U= 632.5, p < 0.05,
2-tailed).
The average daily travel rate of all 30 porpoises
were significantly different among months (F11, 2980 =
33.337, p < 0.0001), with higher mean travel rates in
March−June (36 km d−1, SD = 26.7, p < 0.0001) com-
pared to the remaining months (22 km d−1, SD =
21.06). The porpoises moved on average twice as fast
in June (53 km d−1, SD = 28) than in all other months
combined (25 km d−1, SD = 6.5).
266
Fig. 4. Average sea ice extent and sea surface temperature (SST) in March in (A) 2013, (B) 2014, (C) 2015 and (D) 2016. Loca-
tions obtained from the tagged porpoises in March for these years are shown with dots for animals tagged in both West Green-
land and Denmark. The area of 0% sea ice (dark blue) covers the same area as sea ice temperatures < 0°C (dark blue)
Nielsen et al.: Behavioural differences between porpoise populations
North Sea harbour porpoises
The 71 harbour porpoises instrumented with satel-
lite transmitters in Danish waters provided infor -
mation for an average of 126 d (range: 5−463 d, SD =
92, Table S1), while covering an MCP area of
588165 km2(Fig. 1). They used most of the Kattegat,
Skagerrak and North Sea except for the southern
part, but the kernel home range (50%) was concen-
trated in the Skagerrak, around Skagen and on the
west coast of Denmark (55° N, Fig. 2). The kernel util-
isation range (95%) displayed their use of the North
Sea and the east coast of the United Kingdom. One
porpoise went north to 64° N and as far west as 3° W,
while 4 animals visited fjords in the southern part of
Norway (south of 63° N).
The average water depth within the total range
of the porpoises tagged in Denmark was 295 m
(range: 1−2188 m, SD = 397), and the tagged por-
poises showed a preference for areas with an aver-
age water depth of 84 m (range: 10−410 m, SD =
69). We found no significant differences in the
average bathymetric depths and months in the
areas used (F11, 272 = 1.487, p = 0.136, Fig. 3B). The
average daily travel rate was significantly different
among months (F11,3292 = 2.478, p < 0.004) and low-
267
Fig. 4. (continued)
Mar Ecol Prog Ser 597: 259–272, 2018
est in April (15 km d−1, SD = 13.1, p < 0.021) com-
pared to all other months (18 km d−1, SD = 16.0).
There were no significant differences (t5805, p =
0.051) in the proportion of months represented in
the tagging data from porpoises tagged in Denmark
and in West Greenland, thus the travel rates for the
2 areas were compared. When including all months,
harbour porpoises from West Greenland had a sig-
nificantly faster travel rate than porpoises from the
North Sea (25 vs. 18 km d−1, t5775, p < 0.0001).
Porpoise distribution in relation to sea ice and SST
The largest extension of sea ice coverage during
2013−2016 in West Greenland occurred in March in
all years (Fig. 4), and was used to analyse the dis-
tance between porpoises (n = 11) and the sea ice
edge. The mean distance from all porpoise positions
to the sea ice was 425 km (range: 10−1336 km, SD =
348), and the mean SST at all porpoise positions in
March was 3.3°C (range: 2.0−11.5°C, SD = 2.91). No
sea ice was present in March in the areas frequented
by the North Sea porpoises, and mean SST was 4.6°C
(range: 5.1−6.9°C, SD = 0.89).
Philopatry
Six of the 15 porpoises from West Greenland that
moved offshore (2 adult and 2 subadult females, and
1 adult and 1 subadult male) returned within 50 km
of their tagging position after a mean of 490 d after
tagging (range: 344−741, SD = 193). Five of these
returned the following summer (July−August, after a
mean of 366 d after tagging), and 1 subadult male
porpoise returned to West Greenland in July 2016,
2 yr after deployment, after visiting the East Green-
land shelf during July 2015 in the first year after
instrumentation. Another offshore male porpoise
made extensive movements in the North Atlantic (as
far east as 22° W) before returning to West Greenland
737 d after tagging. This male left the tagging area
for a second time, in October, 81 d upon its latest
arrival. The remaining 9 porpoises did not return to
the tagging area, but ended their transmissions after
a mean of 211 d (range: 29−487, SD = 155). Seven of
the 9 porpoises ended their transmissions between
October and March (29−268 d after instrumentation).
One of the 9 porpoises (subadult male) arrived at the
East Greenland shelf areas in September 2015, 1 yr
after instrumentation, where it stayed until con tact
with the tag was lost in November 2015. The last of
the 9 porpoises commenced its return to West Green-
land at 50° N, but ended the transmissions ~300 km
before reaching the tagging area.
Six male porpoises (5 subadults and 1 adult) from
the North Sea returned within 50 km of their tagging
position in March (April−June), on average 372 d
after tagging (range: 300−461 d, SD = 59). The re -
maining 65 porpoises transmitted for a mean 42 d
(range: 1−215 d, SD = 35).
DISCUSSION
This study is the first to document harbour por-
poises leaving shelf areas to conduct large-scale
wintering movements over deep waters in the North
Atlantic, where some moved as far east as 18° W
(more than 2000 km from the tagging area). The por-
poises tagged in West Greenland used an area 7.5
times larger than the area used by porpoises from the
North Sea. Both the scale of the movements and the
seasonal selection of offshore habitats are evidently
different from what is typical for tracked porpoises in
the North Sea or the Northwest Atlantic, which move
in more focal areas (Read & Westgate 1997, Johnston
et al. 2005). Due to their size and limited capacity for
energy storage, harbour porpoises are believed to
spend the majority of their time locating suitable
prey items; thus their movement is most likely a
direct consequence of the location of preferred prey
(Gannon et al. 1998, Sveegaard et al. 2015, Wisni -
ews ka et al. 2016, 2018). In winter, when the sea ice
covers the majority of the Davis Strait and parts of the
Labrador Sea, porpoises are forced to abandon the
shelf areas of West Greenland until spring, when the
winter sea ice retreats. This retrieval triggers a large
bloom of primary production that attracts high densi-
ties of phytoplankton-foraging fish and zooplankton
(Bluhm & Gradinger 2008) on which West Greenland
porpoises and other cetaceans forage.
This study has documented an impressive seasonal
movement of harbour porpoises from West Green-
land which has not previously been reported for por-
poises, although it has been suggested that the Gulf
of Maine/Bay of Fundy porpoise population has the
capacity to make extensive offshore winter move-
ments in order to follow their prey (Read & Gaskin
1985, Palka et al. 1996, Read et al. 1996). This might
also be true for other porpoise populations, and the
lack of documented offshore sightings of porpoises in
the North Atlantic may be due to lack of long-term
tracking studies and a seasonal mismatch in survey
coverage and porpoise distribution. Surveys for har-
268
Nielsen et al.: Behavioural differences between porpoise populations
bour porpoises in the North Atlantic have always
been conducted during summer, at which time por-
poises are located in shelf waters as indicated by this
study (in particular in July and August, see Fig. 1);
thus there have been no observations of their winter
distribution. Satellite telemetry offers a unique possi-
bility for studying movements and behaviour of spe-
cies that are overlooked due to their cryptic nature,
such as harbour porpoises.
The porpoises tagged in Danish waters were
instrumented with satellite transmitters designed to
detach by using iron nuts that corrode with time
(Teilmann et al. 2007, Sveegaard et al. 2011). This
could potentially have caused the tags to detach
before the batteries were drained, consequently pro-
viding less information. In West Greenland, stainless
steel nuts were used to obtain data on inter-annual
movements. Some of the tags attached to porpoises in
West Greenland provided a record duration of 1047 d
and provided novel insight into porpoise movements
over several years, revealing their affiliation to the
shelves of Greenland.
The harbour porpoises from West Greenland that
transmitted during winter/spring moved into off-
shore areas with significantly greater water depths
than where they spend the summer/autumn. This is
in contrast to the North Sea porpoises, which showed
no seasonal selection of areas with specific water
depths (Fig. 3). This is probably due to general avail-
ability of ice-free shallow water depths in the North
Sea that allow the porpoises to forage on both pelagic
and benthic species, providing a larger prey base
with less need for extensive movements.
There are no records of harbour porpoises being
present in areas with sea ice formation, and their
small size and dorsal fin probably makes them vul-
nerable to ice; however, harbour porpoises in West
Greenland seem unaffected by negative SST and
were distributed as close as 10 km from the sea ice.
In general, the distance of porpoises to the sea ice
varied greatly in this study; some animals were dis-
tributed close to the edge of the sea ice but never
inside areas with ice, while others moved away and
went south into the North Atlantic (Fig. 4). The
SSTs used by the porpoises from West Greenland
(−2 to 11.5°C) suggest a large tolerance towards a
wide range of SSTs rather than having a preferred
SST, which is in contrast to the study by Wingfield
et al. (2017).
Two female porpoises tagged in West Greenland
showed the capacity to perform deep dives (390 and
410 m, respectively) to nearly twice the maximum
depths previously reported for harbour porpoises
(Westgate et al. 1995, Teilmann et al. 2007). This sug-
gests that harbour porpoises from West Greenland
are specialised in using a different niche than other
harbour porpoise populations, and they probably rely
on mesopelagic prey resources. Little is known about
the mesopelagic community in offshore waters,
despite the large estimated fish biomass (St. John et
al. 2016), but it is likely that the harbour porpoises
from West Greenland target this zone during winter.
This is further supported by the binned maximum
dive depths (Mk10 tags) that indicate the potential
for harbour porpoises reaching the mesopelagic layer
at 200 m, and also by findings of vertically migrating
lanternfish in the stomachs of West Greenland por-
poises (Heywood 1996, Heide-Jørgensen et al. 2011).
In addtion, findings of several mesopelagic species in
the stomach of an immature male porpoise bycaught
in an offshore driftnet on the US east coast support
this hypothesis (Read et al. 1996).
Porpoises wintering in the North Atlantic and per-
haps also in West Greenland are likely adapted to
deal with larger spatial dispersal and more dynamic
behaviour of high-quality prey species, which
require more ex tensive movements compared to
Northeast Atlantic porpoises. This suggests that por-
poises from West Greenland in comparison to por-
poises from the North Sea are challenged by a more
geographically dispersed prey field, both vertically
and horizontally. This is supported by the higher
daily travel rate in porpoises from West Greenland in
the months when they are present in offshore waters
(Fig. 3A). The travel rate of porpoises from the North
Sea is within the range for porpoises located on shelf
areas in West Greenland, whereas the larger daily
travel rate covering larger areas for offshore por-
poises suggests that these porpoises are more chal-
lenged in their search for prey and therefore have a
larger daily search radius. The lower travel rate seen
in April for the North Sea porpoises probably reflects
less in tensive foraging in this period where the water
starts warming up and the animals need less energy,
or potentially caused by a seasonal shift in prey as
seen in other porpoise populations (Gannon et al.
1998, Víkingsson et al. 2003).
Porpoises in Greenland mate and give birth during
summer (Lockyer et al. 2003b), and site fidelity is an
important driver for the porpoises, as demonstrated
in our study. This is supported by a subadult male
porpoise (PTT ID 372281, Table 1) that spent the first
post-deployment summer (July) in East Greenland,
but returned to West Greenland for the second sum-
mer (July +1 yr), where it presumably became sexu-
ally mature (at ca. 127 cm according to Lockyer et al.
269
Mar Ecol Prog Ser 597: 259–272, 2018
2001). This is also reflected in the kernel density esti-
mations, where the 50% core area is at the shelves
off Greenland (Fig. 2). Offshore porpoises are all
moving in areas with deep waters in May and start
returning to West Greenland in June and July, and in
August, most porpoise positions are located in either
East or West Greenland (Fig. 1). This could also ex -
plain why the porpoises moved nearly twice as fast in
June compared to all other months as they returned
to West Greenland to reproduce.
Fifty percent (n = 15) of the porpoises tagged in
West Greenland moved offshore after an average of
106 d; however, sex and length of the remaining 15
animals that stayed in the continental shelf area did
not indicate any segregation in behaviour that could
explain their avoidance of offshore areas. Rather, the
remaining porpoises probably did not transmit long
enough to document offshore movement, as seen for
the 8 tags with defective batteries that reduced the
longevity of the tag.
In order to identify the movement potential and site
fidelity to the summer feeding grounds of harbour
porpoises from West Greenland, the tag has to trans-
mit for at least 2 yr, as some animals did not return to
West Greenland until >700 d after deployment. How-
ever, this may be due to the animal being immature
during the second summer and therefore not needing
to return to West Greenland to reproduce until the
third summer.
This study demonstrates that the population of har-
bour porpoises in West Greenland disperses wider
than other harbour porpoise populations for which
movements have previously been studied. Harbour
porpoises from West Greenland are capable of in -
habiting deep oceanic waters of the North Atlantic
and perform deep dives. The tracking also indicates
that despite having wide-ranging dispersal capabil-
ity, harbour porpoises with transmitters lasting >1 yr
exhibited site fidelity to specific summer feeding
grounds, as also suggested by Read & Westgate (1997)
for harbour porpoises in Canadian waters, and seen
in porpoises from the North Sea (this study). Five of
the 6 porpoises that returned to West Greenland and
to the very same place where they were tagged,
were presumed to be sexually mature (PTT ID 21791
was 111 cm when tagged and could most likely not
have reached sexual maturity upon its return, 344 d
after deployment), which suggests that this area is an
important feeding, breeding and mating ground dur-
ing summer and fall.
Site fidelity to the summering area explains the
genetic and morphometric differentiation of the West
Greenland population from other harbour porpoise
populations in the North Atlantic (Andersen et al.
2001, Tolley et al. 2001, Huggenberger et al. 2002).
However, independent of the genetic differentiation,
the West Greenland porpoises also display ecological
specialisations compared to other studied harbour
porpoise populations (Fontaine et al. 2014) by dis-
playing a large prey diversity (Heide-Jørgensen et al.
2011) and different migratory and diving behaviour,
all characteristics suggesting that harbour porpoises
from West Greenland belong to a unique oceanic
ecotype.
Acknowledgements. We thank Mikkel V. Jensen for modi-
fying the tags for instrumentation on the harbour porpoises
and the hunters Svend and Knud Heilmann for helping
with capture and tagging of the porpoises. The Danish
Meteorological Institute is thanked for providing ice charts
maps. The project was supported by the Greenland Min-
istry of Education, Church, Culture & Gender Equality, the
Danish Cooperation for the Environment in the Arctic
(DANCEA, Danish Ministry of the Environment) and the
Greenland Institute of Natural Resources. The study was
performed with permission from the Government of
Greenland, permit no. 2012-069733, Doc. 1265044. The
Danish porpoises were tagged with permission from the
Danish Forest and Nature Agency (SN 343/SN-0008, SN
2001-404-0001, NST-3446-0016) and the Animal Welfare
Division (Ministry of Justice, 1995-101-62, 2010-561-1801,
2015-15-0201-00549). The Danish Forest and Nature
Agency supported the project. The collaboration with the
Danish pound net fishermen and a large number of field
workers is gratefully acknowledged. Finally, 4 reviewers
are greatly acknowledged for their valued comments and
improvement of this paper.
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Editorial responsibility: Peter Corkeron,
Woods Hole, Massachusetts, USA
Submitted: November 29, 2017; Accepted: April 4, 2018
Proofs received from author(s): May 20, 2018
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The harbor porpoise (Phocoena phocoena) is common in temperate waters of the eastern North Pacific Ocean, including Southeast Alaska inland waters, a complex environment comprised of open waterways, narrow channels, and inlets. Two demographically independent populations are currently recognized in this region. Bycatch of porpoises in the salmon drift gillnet fisheries is suspected to occur regularly. In this study, we apply distance sampling to estimate abundance of harbor porpoise during ship surveys carried out in the summer of 2019. A stratified survey design was implemented to sample different harbor porpoise habitats. Survey tracklines were allocated following a randomized survey design with uniform coverage probability. Density and abundance for the northern and southern Southeast Alaska inland water populations were computed using a combination of design-based line- and strip-transect methods. A total of 2,893 km was surveyed in sea state conditions ranging from Beaufort 0 to 3 and 194 harbor porpoise groups (301 individuals) were detected. An independent sighting dataset from surveys conducted between 1991 and 2012 were used to calculate the probability of missing porpoise groups on the survey trackline (g[0]=0.53, CV=0.11). Abundance of the northern and southern populations were estimated at 1,619 (CV=0.26) and 890 (CV=0.37) porpoises, respectively. Bycatch estimates, which were only obtained for a portion of the drift gillnet fishery, suggest that mortality within the range of the southern population may be unsustainable. Harbor porpoises are highly vulnerable to mortality in gillnets, therefore monitoring abundance and bycatch is important for evaluating the potential impact of fisheries on this species in Southeast Alaska.
... Harbor porpoises (Phocoena phocoena) are charismatic and protected animals, with a broad societal, scientific, and political interest in their health and wellbeing (Evans and Hammond, 2004;Peltier et al., 2013;Evans, 2019). Harbor porpoise habitat includes both offshore and nearshore waters of the Northern Hemisphere (Evans and Hammond, 2004;Hammond et al., 2017;Nielsen et al., 2018). The harbor porpoise is currently listed as 'least concern' by the International Union for Conservation of Nature given their large global population size, which is likely well over a million individuals (Braulik et al., 2020). ...
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Humans impact natural systems at an unprecedented rate. The North Sea is one of the regions in the world with the highest levels of anthropogenic activity. Here, the harbor porpoise (Phocoena phocoena) is an abundant species and is often regarded as an ecosystem sentinel. A post-mortem surveillance program was established in the Netherlands aimed at increasing knowledge of the effects of human activities on harbor porpoises. In this study, we describe the pathological findings related to anthropogenic and natural causes of death categories in 612 harbor porpoises that stranded between 2008 and 2019, and assess their relations to age, sex, season, and location. The largest anthropogenic category was bycatch (17%), with mainly juveniles affected and peak periods in March and September–October. Other, infrequently diagnosed anthropogenic causes of death were trauma (4%), largely most likely due to ship collisions, and marine debris ingestion and entanglement (0.3%). The risk of dying from anthropogenic causes was highest for juveniles. Lesions compatible with noise-induced hearing loss were investigated in carcasses which were fresh enough to do so (n = 50), with lesions apparent in two porpoises. Non-direct human-induced threats included infectious diseases, which were by far the largest cause of death category (32%), and affected mainly adults. Also, gray seal (Halichoerus grypus) attacks were a frequently assigned cause of death category (24%). There were more acute predation cases in the earlier study years, while porpoises with lesions that suggested escape from gray seal attacks were diagnosed more recently, which could suggest that porpoises adapted to this threat. Our study contributes to understanding porpoise health in response to persisting, new, emerging, and cumulative threats. Building up such knowledge is crucial for conservation management of this protected species
... They usually forage near the sea bottom, but may also do some pelagic feeding at night (Bjørge & Tolley, 2018). Recently, studies have reported that, at least in one population, individuals perform seasonal travels to offshore temperate waters where they feed on mesopelagic fish at depths around 400 m (Nielsen et al., 2018;Nielsen et al., 2019). In contrast, P. dalli prefers deep offshore waters of the North Pacific, but is also found in deep nearshore and inshore waters along the west coast of North America. ...
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... For harbour porpoises in the inner Danish waters, where continuous foraging takes place at night in depths primarily less than 25 m [21,30], energy gain may be maximized by performing shorter dives with a moderate ƒ H allowing continued foraging while digesting. However, we predict that porpoises targeting deep-water/mesopelagic prey, such as in Greenlandic waters where porpoises dive down to 410 m [48,49], will exhibit a graded and occasionally extreme diving bradycardia with extended post-dive intervals after the longest dives. ...
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... The tracking study also showed-quite unexpected-that the harbour porpoises undertook long travels in offshore Atlantic waters (APNN, unpubl. data, Nielsen et al. 2018). ...
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Elucidating the evolutionary and ecological characteristics of distinct populations constitutes a cornerstone in the classification of ecotypes, and in assessing their specific responses to environmental changes and potential impacts from human activities. In this study, two complementary approaches were deployed to investigate the existence of a putative harbour porpoise (Phocoena phocoena) ecotype in West Greenland. Genetic differentiation of 68 porpoises from West Greenland, and neighbouring Canada and Iceland were studied by ddRADseq analysis, and 18 porpoises instrumented with satellite transmitters were used to study their movement behaviour and site fidelity. The results suggest a genetically distinct harbour porpoise population in West Greenland, with strong site fidelity during the August breeding period and wide-ranging dispersal in the North Atlantic at other seasons. This adds to previously described unique characteristics of West Greenland harbour porpoises, including mesopelagic foraging behaviour, distinct skull morphology and tooth ultrastructure, and shorter, yet heavier, body; all pointing to the existence of a distinct West Greenland ecotype. We hypothesize that this ecotype arose through gradual adaptation to the local environmental conditions of the West Greenlandic shelf area, including high summer primary productivity and seasonal sea ice coverage. Consequently, this distinct ecotype of harbour porpoises necessitates a focused conservation plan.
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The question of how individuals acquire and allocate resources to maximize fitness is central in evolutionary ecology. Basic information on prey selection, search effort, and capture rates are critical for understanding a predator's role in its ecosystem and for predicting its response to natural and anthropogenic disturbance. Yet, for most marine species, foraging interactions cannot be observed directly. The high costs of thermoregulation in water require that small marine mammals have elevated energy intakes compared to similar-sized terrestrial mammals [1]. The combination of high food requirements and their position at the apex of most marine food webs may make small marine mammals particularly vulnerable to changes within the ecosystem [2-4], but the lack of detailed information about their foraging behavior often precludes an informed conservation effort. Here, we use high-resolution movement and prey echo recording tags on five wild harbor porpoises to examine foraging interactions in one of the most metabolically challenged cetacean species. We report that porpoises forage nearly continuously day and night, attempting to capture up to 550 small (3-10 cm) fish prey per hour with a remarkable prey capture success rate of >90%. Porpoises therefore target fish that are smaller than those of commercial interest, but must forage almost continually to meet their metabolic demands with such small prey, leaving little margin for compensation. Thus, for these "aquatic shrews," even a moderate level of anthropogenic disturbance in the busy shallow waters they share with humans may have severe fitness consequences at individual and population levels.
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From the Gulf of Maine/Bay of Fundy harbour porpoise Phocoena phocoena population, the average abundance was 47 200 for 1991 and 1992. Based on observer programs, in 1993 the bycatch in the groundfish sink gillnet fishery from the US Gulf of Maine was 1400 and 200 to 400 animals from the Canadian Bay of Fundy. From the Gulf of St. Lawrence, no abundance estimates are available, and based on mail surveys conducted in 1989 to 1991, approximately 2800 harbour porpoises were caught annually in fishing gear. Reliable abundance and bycatch estimates are not available from the Newfoundland population. Information on life history and reproduction parameters, diet and contaminant concentrations are also provided.
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